1. Field of the Invention
This invention pertains to an analog memory driver for all classes of light emitting devices where the light output is a function of current. The analog memory driver is a memory unit and driver where the current through the display device is controlled by an analog voltage which is set from an analog drive line using a sample and hold circuit.
2. Description of the Prior Art
Well-designed current LED drivers currently use a constant current drive to compensate for variations in the forward voltage drop of various LEDs, and where the current is set by operating voltages or with current regulators, and the intensity of the LED is controlled by pulse width modulation. The overall intensity of the display may be varied by either selecting alternate pulse width time periods, or by deleting small time segments of the LEDs that have been activated. The displays used for these video systems use eight bits to define the intensity for each of the red, blue and green LEDs which give 256 intensity levels for each of the three colors for a total of 16,777,216 color combinations. To accomplish this with a pulse width modulated system requires that the screen face be refreshed eight times with variable display intervals for each field within the frame time of standard video of 30 frames per second. While 30 frames per second is adequate for phosphor based video displays, it is not adequate for LED displays, and typically 120 frames per second must be used to remove the viewing artifacts when using instantaneous light emitting devices. This is a very difficult task for video based display systems of 320 by 256 pixels or larger and requires multiple processors to accomplish the task.
Prior art patents in this field include U.S. Pat. No. 4,659,967 issued on Apr. 21, 1987 to Dahl; U.S. Pat. No. 5,111,195 issued on May 5, 1992 to Fukuoka et al.; U.S. Pat. No. 5,250,939 issued on Oct. 5, 1993 to Takanashi; U.S. Pat. No. 5,325,106 issued on Jun. 28, 1994 to Bahraman; U.S. Pat. No. 5,363,118 issued on Nov. 8, 1994 to Okumura; U.S. Pat. No. 5,426,430 issued on Jun. 20, 1995 to Schlig; U.S. Pat. No. 5,523,772 issued on Jun. 4, 1996 to Lee; U.S. Pat. No. 5,572,211 issued on Nov. 5, 1996 to Erhart et al.; U.S. Patent No. 5,574,475 issued on Nov. 12, 1996 to Callahan, Jr. et al. and U.S. Pat. No. 5,633,651 issued on May 27, 1997 to Carvajal et al.